We validate that changes of ground deformation recorded by GPS contain useful information for earthquake forecasting. A moving rate of variation filter is used to extract short-term signals from GPS time series in New Zealand, California, and Japan. The precursory information of these signals for large earthquakes is evaluated using Molchan's error diagram. The results suggest that the GPS signals provide a probability gain of 2–4 for forecasting large earthquakes against a Poisson model. Further tests show that the GPS signals are not triggered by large earthquakes, and that the probability gain is not derived from forecasting aftershocks. This demonstrates that noncatalog information, such as GPS data, can be used to augment probabilistic models based on seismic catalog data to improve forecasting of large earthquakes.